Spatial proteomics reveals lipid droplet reorganization in symbiotic Paramecium bursaria cells

Endosymbiosis is an important adaptive mechanism allowing organisms to exploit novel niches. The flexible relationship between the ciliate Paramecium bursaria and green algae represents a model system for studying early endosymbiosis evolution. However, the mechanisms underlying how P. bursaria cells maintain this endosymbiotic relationship remain unclear. Here, we present a spatial proteomics atlas of P. bursaria cells with or without endosymbionts. We reveal that lipid droplets are reorganized in symbiotic hosts, with many proteins undergoing subcellular translocation. Consistent with our proteomics data, we show that the morphology and localization of lipid droplets changes near endosymbionts in host cells. Perturbing symbiotic cells with chemical inhibitors of lipid metabolism reduces endosymbiotic algal stability, revealing an involvement of lipid droplets in the host-endosymbiont interaction. Our study provides a comprehensive resource on protein localization and translocation in P. bursaria cells and elucidates how those cells remodel an existing organelle to maintain endosymbiosis.